linux-kernel - Re: [PATCH v2 1/2] tty: add missing rcu_read_lock for task

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Message-ID: <CACh33FrCn+pDPNLVnrYVQ5sf92m8hj+oiQJ2U6RM2CtqDBf=OQ@mail.gmail.com>
Date:	Sun, 28 Jun 2015 13:20:34 -0400
From:	Patrick Donnelly <batrick@...bytes.com>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] tty: add missing rcu_read_lock for task_pgrp

On Sun, Jun 28, 2015 at 11:23 AM, Peter Hurley <peter@...leysoftware.com> wrote:
> On 06/27/2015 08:51 PM, Patrick Donnelly wrote:
>> task_pgrp requires an rcu or tasklist lock to be obtained if the returned pid
>> is to be dereferenced, which kill_pgrp does. Obtain an RCU lock for the
>> duration of use.
>
> kill_pgrp() obtains tasklist_lock, so I don't see an unsafe deref.

I see a race between looking up the pgrp via task_pgrp and passing it
to kill_pgrp. The pgrp struct pid may be freed via setpgid/setsid, as
mentioned in the comment for task_pgrp:

 * Without tasklist or rcu lock it is not safe to dereference
 * the result of task_pgrp/task_session even if task == current,
 * we can race with another thread doing sys_setsid/sys_setpgid.

Getting the lock after the lookup is getting the lock too late. I
could be wrong though as I'm no expert on locking in Linux.

-- 
Patrick Donnelly
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